This invention relates to the art of operating fluidized bed reactors and more particularly to an improved method of operating fluid bed reactors equipped with steam coils so as to reduce the thermal shock to which the coils are subjected when brought into service.
Various forms of fluidized bed chemical processing units incorporate coils for cooling or heating purposes. Fluidized bed processing units of the type wherein an exothermic chemical reaction occurs in the bed often employ immersed cooling coils to generate saturated steam. As is well known, even in a continuous process situation a fluidized bed reactor will undergo load changes, e.g., during startup or as a consequence of changes in feedstock composition, etc.. During such load changes it is desireable to modulate the steam generation rate to maintain thermal equilibrium in the reactor. However, the heat transfer coefficient for the cooling coils is largely independent of the fluid dynamic conditions, and the bed side of the cooling coils is substantially insensitive to changes in the fluidizing velocity. A number of methods have been used to modulate the steam generation rate. One method is to slump a section of the bed to increase the bed side heat transfer resistance. Another method is to lower the bed temperature to reduce the driving force. In many processes the foregoing methods are of limited value or unacceptable. Another more common method is to arrange the cooling coils in groups so that the water supply can be cut off to selected coils, in which event the coil will boil off to the dry state and come to bed temperature. That particular method allows steam generation to be varied by discrete steps over a wide range.
However, cycling steam generation coils into and out of service subjects them to thermal shock. Once water flow is initiated the cooling tube wall temperature rapidly drops from the bed temperature to a value near the water temperature, because the heat transfer resistance on the bed or outer side of the cooling tube wall is far greater than it is at the water or inner side of the tube wall. As a result the cooling tube wall is subjected to thermally induced stress. The magnitude of the developed stress is proportional to the temperature change and has a substantial impact on the number of start-up cycles a coil can withstand before it fails. It is necessary to reduce the possibility of coil failure since this in turn can cause overpressurization of the containing vessel and create a hazardous condition for the operating personnel. Frequently the temperature change is as great as 400 to 800 degrees F. and in such cases the useful life of the coil is greatly curtailed as a consequence of the stresses developed during start-up.